If you want to Dual Boot Windows 7 and Windows 8 and have already successfully set this up you will find that Windows 8 is the default option to start up.
You can in the blue Windows start screen opt for either Windows 7 or Windows 8 as the default.
Changing to Windows 7 you will on the next start get a DOS black screen with the same option to select but not the option to change the default.
If you go to Run and type _msconfig in Windows 7 and then delect Boot_you can set the default and the timeout.
Be careful as you can also delete an option to boot the other Windows start option here too.